Semiconductor wafers are generally prepared from a single crystal ingot (e.g., a silicon ingot) which is trimmed and ground to have one or more flats or notches for proper orientation of the wafer in subsequent procedures. The ingot is then sliced into individual wafers. While reference will be made herein to semiconductor wafers constructed from silicon, other materials may be used to prepare semiconductor wafers, such as germanium, silicon carbide, silicon germanium, or gallium arsenide.
Semiconductor wafers (e.g. silicon wafers) may be utilized in the preparation of composite layer structures. A composite layer structure (e.g., an SOI structure) generally comprises a carrier wafer or layer, a device layer, and an insulating (i.e., dielectric) film (typically an oxide layer) between the carrier layer and the device layer. Generally, the device layer is between 0.05 and 20 micrometers thick. In general, composite layer structures, such as silicon on insulator (SOI), silicon on sapphire (SOS), and silicon on quartz, are produced by placing two wafers in intimate contact, followed by a thermal treatment to strengthen the bond.
After thermal anneal, the bonded structure undergoes further processing to remove a substantial portion of the donor wafer to achieve layer transfer. For example, wafer thinning techniques, e.g., etching or grinding, may be used, often referred to as back etch SOI (i.e., BESOI), wherein a silicon wafer is bound to the carrier wafer and then slowly etched away until only a thin layer of silicon on the carrier wafer remains. (See, e.g., U.S. Pat. No. 5,189,500, which is incorporated in its entirety herein by reference.) This method is time-consuming and costly, wastes one of the substrates and generally does not have suitable thickness uniformity for layers thinner than a few microns.
Another common method of achieving layer transfer utilizes a hydrogen implant followed by thermally induced layer splitting. Particles (e.g., hydrogen atoms or a combination of hydrogen and helium atoms) are implanted at a specified depth beneath the front surface of the donor wafer. The implanted particles form a cleave plane in the donor wafer at the specified depth at which they were implanted. The surface of the donor wafer is cleaned to remove organic compounds deposited on the wafer during the implantation process.
The front surface of the donor wafer is then bonded to a carrier wafer to form a bonded wafer through a hydrophilic bonding process. The donor wafer and/or carrier wafer are activated by exposing the surfaces of the wafers to plasma containing, for example, oxygen or nitrogen. Exposure to the plasma modifies the structure of the surfaces in a process often referred to as surface activation, which activation process renders the surfaces of one or both of the donor water and carrier wafer hydrophilic. The wafers are then pressed together and a bond is formed there between. This bond is relatively weak, and must be strengthened before further processing can occur.
In some processes, the hydrophilic bond between the donor wafer and carrier wafer (i.e., a bonded wafer) is strengthened by heating or annealing the bonded wafer pair at temperatures between approximately 300° C. and 500° C. The elevated temperatures cause the formation of covalent bonds between the adjoining surfaces of the donor wafer and the carrier wafer, thus solidifying the bond between the donor wafer and the carrier wafer. Concurrently with the heating or annealing of the bonded wafer, the particles earlier implanted in the donor wafer weaken the cleave plane.
A portion of the donor wafer is then separated (i.e., cleaved) along the cleave plane from the bonded wafer to form the SOI wafer. Cleaving may be carried out by placing the bonded wafer in a fixture in which mechanical force is applied perpendicular to the opposing sides of the bonded wafer in order to pull a portion of the donor wafer apart from the bonded wafer. According to some methods, suction cups are utilized to apply the mechanical force. The separation of the portion of the donor wafer is initiated by applying a mechanical wedge at the edge of the bonded wafer at the cleave plane in order to initiate propagation of a crack along the cleave plane. The mechanical force applied by the suction cups then pulls the portion of the donor wafer from the bonded wafer, thus forming an SOI wafer.
According to other methods, the bonded pair may instead be subjected to an elevated temperature over a period of time to separate the portion of the donor wafer from the bonded wafer. Exposure to the elevated temperature causes initiation and propagation of a crack along the cleave plane, thus separating a portion of the donor wafer. This method allows for better uniformity of the transferred layer and allows recycle of the donor wafer, but typically requires heating the implanted and bonded pair to temperatures approaching 500° C. For dissimilar materials (silicon on sapphire or silicon on quartz) this temperature can be too high for the substrates to survive the stresses induced by the mismatch in thermal expansion coefficient. Several methods of lowering the temperature necessary to induce splitting have been discussed in the literature, among them increasing hydrogen dose and co-implanting hydrogen ions and boron ions. Higher doses of H requires longer implant times, leading to higher costs. When co-implanting boron and hydrogen, additional steps may be required to remove the excess boron in the transferred layer since this can lead to undesirable changes to the resistivity of the top layer.